Considerable work has been done recently in the use of induction machines in conjunction with a static inverter. It has been found that a circuit similar to that of a static inverter can be coupled to the output conductors of an induction machine, and when the machine is driven as a generator, the "inverter" acts as a switching system to recycle or redirect the reactive energy from one phase winding to another of the induction machine. In this way the switching system (or inverter circuit) replaces the separate machine for excitation or the capacitor bank previously used. After this first step, it was found that the switching frequency of the thyristors in the inverter circuit could be modulated with respect to the reference or synchronous frequency at which the induction machine is operated to provide an a-c output voltage (with a d-c average level). In implementing this modulated induction generator system, it was found that the machine was derated because, when the system is modulated, the effective a-c square wave output voltage is about half that provided by the machine.
It is therefore a principal object of the present invention to provide a modulated induction generator system with substantially increased efficiency or utilization of the power supplied by the generator.
Another important object of the invention is to provide such an improved system in which the a-c output voltage does not have an average d-c level.
Still another important object of the invention is to produce an inverter circuit for use with the induction generator, which inverter circuit can be energized with a d-c voltage of either polarity, or with an a-c voltage.
A corollary consideration of this invention is to provide such an improved bipolar inverter circuit which has a new commutation circuit arrangement.
A further object of the invention is to provide such an improved bipolar inverter arrangement which finds utility with ac-to-ac power conversion through a heterodyne frequency conversion approach.
Still another important object of the invention is to provide an improved inverter which simplifies the feedback or regeneration of energy through a thyristor bridge toward a source of a-c energy.